OFDM system is widely used in wireless communication such digital audio broadcasts (DAB), digital video broadcasts (DVB), wireless local area network (WLAN) and long term evolution (LTE) system. To guarantee the reliability of the communication, OFDM signal correlation and synchronization in timing and frequency are performed in a receiver. The term “correlation” refers to an operation of detecting similarity between two signal samples. The term “synchronization” refers to all operations for detecting similarity between two signal samples that includes controlling data flow, scheduling tasks, sampling and correlating the signal samples. Correlation is a part of synchronization process.
One of the common practices in timing and frequency synchronizations is to transmit a pair of redundant synchronization sequence along with data. The sequences are correlated at a receiving side to detect a starting point of receiving data. Generally, timing and frequency synchronization is accomplished at multiple levels. The synchronization may start at superframe level, continue at frame level and complete at symbol level. The length of the synchronization sequence, synchronization window, synchronization delay, data sample rate and other characteristics may vary at every level, but the same synchronization principles apply to all levels.
In general, OFDM signal is organized into hierarchy as shown in FIG. 1. At the top of the hierarchy, a signal sequence is grouped into superframes 100. A superframe 100 may have a superframe header 101 and a predetermined number of frames 102. A frame 102 may have a frame header (HD) 104 and a predetermined number of symbols 106. The header 104 comprises of a frame identifier (FID) 108, a pair of redundant synchronization (sync) sequences 110, and may have other data interval 112 in between. The two identical sync sequences 110 are correlated in a receiver to detect the starting point of a frame 102. Symbol 106 comprises of cyclic prefix (CP) 114 and data payload 116. Data payload 116 includes regular data 118 and cyclic prefix data 114a. Cyclic prefix 114 is an exact replication of cyclic prefix data 114a. The two identical cyclic prefix 114 sequences are correlated in a receiver to detect the starting point of a symbol 106. Cyclic prefix 114 also serves as a guard interval to prevent interference between neighboring symbols. The coarse timing and fine frequency synchronizations of OFDM signal are accomplished by correlating sync sequences 110 at frame level and correlating cyclic prefixes 114 at symbol level.
FIG. 2 shows the timing of frame header 104 and cyclic prefix 114 in a superframe 100. Header 104 and cyclic prefix 114 consume about 15% or less of the full bandwidth. Synchronization process starts when a valid signal is detected and continues until the end of data stream. In general, sync sequences 110 are correlated first to detect the beginning of a frame 102. Then, cyclic prefixes 114 are correlated to detect the beginning of every symbol 106 in a frame 102. The cyclic prefix 114 correlations continue until all symbols 106 in a frame 102 are processed. In conventional practices, due to different in synchronization window size, synchronization delay and synchronization sequence length at frame and symbol levels, dedicated devices are implemented for synchronization at each level. However, in mobile communication where cost is highly competitive, it is desirable to provide a single integrated device to execute synchronizations at all levels to reduce component cost and power consumption.
Further, in conventional practices, due to uncertainty of header 104 and cyclic prefix 114 timing; synchronization and correlation are kept active during whole data packet cycle. This practice may not be efficient in power consumption during synchronization, since correlation is only part of the synchronization process. Therefore, it is desirable to power off the correlation circuits to reduce power consumption when data correlation is complete during synchronization.
Further, in the known prior art, data sample rate for correlation is predetermined to be equivalent to the incoming data rate. In general, power consumption is proportion to data sample rate, and sample quality (signal to noise ratio S/N) is related to channel conditions that change dynamically. Therefore, to reduce power consumption, it is desirable to decrease the sample rate when channel S/N is high and increase the sample rate when the channel S/N is low dynamically during synchronization.
Hence, it is an object of this invention to provide an improved method and system that is capable to process multiple levels of timing and frequency synchronizations with a single integrated device and to control the power consumption of the system according to correlation and synchronization activities. It is another object of this invention to provide an improved method and system that reduce system power consumption by changing correlation data sample rate dynamically according to communication channel conditions.